Run this to see the biggest folders on the root filesystem:

du -xh --max-depth=1 / 2>/dev/null | sort -rh | head -20

That shows the top 20 space hogs. Paste the output here and I'll tell you exactly what to delete.

While that runs, the safest immediate frees you can do right now (won't break anything):

sudo apt clean
sudo apt autoremove -y
sudo journalctl --vacuum-time=3d
rm -rf ~/.cache/pip

Then re-check df -h. But run the du command first so we can see the real culprit — a 13G SD card filling up is usually one or two big things (old kernels, Docker, logs, or a large file).
